Preamplifier circuits are used in numerous applications. Typically, a preamplifier is used in an electronic system to increase the amplitude of a weak signal created within the electronic system. A singled ended preamplifier amplifies a single ended input signal by a gain factor such that the output signal is equal to the input signal multiplied by the gain factor. Differential preamplifiers are a particular type of preamplifier wherein the differential input signal comprises a positive rail component and a negative rail component. The preamplifier increases the amplitude of both input signal components by a gain factor. The ability of the output signal of a preamplifier circuit to faithfully reproduce the input signal is a function of many factors, including among other factors, the bandwidth of the preamplifier, the frequency of the input signal, the impedance of the input system and transmission line impedance that provides the input signal, and the input impedance of the preamplifier.
In order to obtain a relatively constant gain over the specified bandwidth of a preamplifier, it is desirable to match the input impedance of the preamplifier and the impedance of the input system and transmission line impedance that provides the input signal to preamplifier. If a significant mismatch exists between the input impedance of the preamplifier and the system providing the input signal, signal reflections at the input to the preamplifier, caused by the impedance mismatch, will compromise the performance of the preamplifier. In such a situation, the output signal will be degraded or noisy at signal transitions, consequently narrowing the bandwidth over which the system can effectively operate.
The impedance of the system providing the input signal can vary significantly according to the particular system and application. In a disk drive system, the electrical resistance of a magneto-resistive (MR) head changes in response to variations in magnetic flux. The MR head is moved over the surface of a disk in order to read the data stored on the disk. The MR head reads data from a disk by sensing flux changes (e.g., changes in polarity) on the magnetic surface of an associated disk as the magnetic surface passes beneath the MR head. The flux change causes a change in the resistance of the MR head. As a function of the change in resistance, the MR head provides a corresponding change in voltage. This voltage is provided to the input of a differential preamplifier which amplifies the signal for use by other components in the system.